«The ground calibrations of the WFC3/UVIS G280 grism H. Kuntschner, H. Bushouse, M. Kümmel, J. R. Walsh January 22, 2009 ABSTRACT Based on thermal ...»
ST-ECF Instrument Science Report WFC3 2009-01
The ground calibrations of
the WFC3/UVIS G280 grism
H. Kuntschner, H. Bushouse, M. Kümmel, J. R. Walsh
January 22, 2009
Based on thermal vacuum tests (TV2; June - August 2007 and TV3; March/April
2008), the performance of the WFC3 UV G280 grism has been assessed. The
locations of the different orders relative to exposures taken through a direct
imaging filter are determined, trace and wavelength solutions are derived for a central position on each chip, and the absolute throughput of the different orders is quantified. Aperture corrections are given as a function of wavelength.
Furthermore, we describe flat-field cubes that provide pixel-to-pixel information as a function of wavelength to an accuracy of about 2%.
The +1st order is useful for scientific observations in the range ~190 to 500nm, however, the +2nd order spectrum is overlapping with the first order for wavelengths larger than about 390nm. For wavelengths above ~320nm, the 0th order, which shows a small dispersion, carries more power than the +1st order.
Therefore, especially for red sources, the 0th order is prone to saturation effects.
The trace and wavelength solutions show significant variations as a function of position within the field-of-view. However, the available ground calibrations are not sufficient to establish a full position-dependent calibration. Henceforth, the calibrations reported in this ISR can only be used to extract spectra of relatively isolated, blue targets placed at the central position on Chips 1 or 2.
The Space Telescope European Coordinating Facility. All Rights Reserved.
ST-ECF Instrument Science Report WFC3-2009-01 1. Introduction The Wide Field Camera 3 (WFC3) is fitted with three grisms for slitless spectroscopy. In the UVIS channel there is one grism, G280, for the near-UV to visible range (200 - 400nm). The NIR channel has two grisms (G102 and G141) for the shorter (800 - 1150nm) and longer NIR wavelengths (1100-1700nm).
The fundamental design parameters of a grism are the deflection of the incident beam by the grism (defined by the prism angle), dispersion in the various orders and the energy in each order (defined by the groove frequency and profile). In order to extract slitless spectra from grism images it is necessary to know these parameters and their variation with position in the field. With a good parameterization, then extraction software, such as the aXe task developed at the ST-ECF (http://www.stecf.org/software/slitless_software), can be applied to extract multiple slitless spectra from sky images in a semi-automatic fashion. A further use of the parameterization is to provide a simulation package that can predict twodimensional dispersed images of a given sky region if an input catalogue is provided. The ST-ECF has developed such a package for Cycle 17 (aXeSIM;
Kümmel, Kuntschner, Walsh 2007; Kümmel et al. 2009).
During the Thermal Vacuum testing of WFC3 in June - August 2007 (TV2) and March/April 2008 (TV3), specific calibrations for the UVIS grism were included.
The tests were designed to allow determination of the spectral trace, the dispersion solution and the absolute instrument throughput by using a combination of monochromator and white light source measurements. Additionally, the detector flat-field was determined with narrowband exposures. This ISR describes the implementation of the tests and the analysis of the results; emphasis is given to the results from TV3 where the flight detectors were installed in WFC3. Tables provide the details of the trace and the dispersion solution; measurements of the total instrument throughput and information on the wavelength dependent flat-field are also provided.
This instrument science report (ISR) is a follow-up on the report by Larsen, Bushouse, & Walsh (WFC3-2005-17) from the TV1 calibrations.
2. Calibration setup in TV2 and TV3 The grism test procedures were executed in TV2 and TV3 with the WFC3 in a flight-like thermal-vacuum environment. The WFC3 external optical stimulus system, CASTLE, was used to provide the necessary source targets for the tests.
For these tests, the WFC3 detector UVIS-2 was used in TV2 and UVIS-1 was used in TV3; the detectors were most of the time at an operating temperature of -83°C.
For the dispersion and spectral trace calibration procedures we used sets of pointsource exposures, all of which were obtained with the CASTLE quartz-tungstenST-ECF Instrument Science Report WFC3-2009-01 halogen (QTH) lamp, and a 10-micron pinhole to provide an unresolved target. An extended source (200 micron pinhole) was used to provide high S/N photometric measurements for the absolute instrument throughput calibrations with the G280 grism in place.
3. Calibration measurements For the UVIS G280 grism, calibration measurements were carried out to allow determination of the spectral trace, the dispersion solution and the absolute instrument throughput as well as a wavelength dependent flat-field. A high level summary of all UVIS grism calibration procedures in TV2 and TV3 is given in Tables 1 and 2, respectively. During TV2 trace and dispersion calibrations were performed in a 5-point pattern (one central position and the four quadrants) to allow for a 2-dimensional solution covering the field-of-view (FoV) of Chip 1+2.
A central position on Chip 1 was calibrated for absolute throughput. In TV3 trace and dispersion calibrations were performed only for one central position on each of the chips (see Table 3). Absolute throughput is determined for one position on each of the chips.
Table 2. Summary of UVIS grism calibrations from TV3 Test procedure Date Grism Purpose UV21S02B 2008-03-18 G280 Absolute throughput Chip 1 UV21S07 2008-04-04 G280 Absolute throughput Chip 2 UV21S04A 2008-04-29 G280 Flat field in Thermal Vac Chip 1+2 UV21S06 2008-04-30 G280 Flat field in ambient Chip 1+2 UV24S07 2008-04-05 G280 Dispersion (centre Chip 1) UV24S08 2008-04-09 G280 Dispersion (centre Chip 2)
ST-ECF Instrument Science Report WFC3-2009-01
The absolute throughput measurements were carried out with a calibrated monochromator source covering wavelengths from 190 to 1000nm (note, that only during TV3 the full wavelength range was covered). The trace and dispersion calibrations were carried out starting with a set of direct image and grism pairs using a white light stimulus. After that monochromator steps covering the range 200-530nm in steps of 15nm for TV2 and 200-915nm in TV3 in increasing steps of 15, 25 and 100nm were performed. In all cases, the monochromator bandwidth was 10 nm.
Additionally, the detector flat-field with full frame illumination was determined in TV2 with 13nm wide bandpasses in steps of 30nm, covering the range 260-530nm;
in TV3 with 13nm wide bandpasses in steps of 30nm, covering the range 260nm with an additional set at 200, 220 and 240nm taken with 16nm wide bandpasses.
4. Analysis In this section we describe the analysis of the calibration data yielding calibrations for the trace and wavelength solutions and the absolute throughput. The fielddependent trace and dispersion data obtained during TV2 for Chip 1+2 showed that the field dependent solutions are complex and cannot be easily expressed in the aXe parameterisation (i.e. surface polynomial fits), at least not with only 5 different field positions available for the calibration (see also Figure 11). Furthermore, during TV3, with the flight detector inserted, only one (central) standard source position could be calibrated for each chip. Therefore, in this ISR we concentrate on the analysis and description of the calibration for these two standard positions (see Table 3) as determined in TV3.
Table 3: Central standard positions on each chip for the trace and dispersion solutions as derived in this ISR from TV3 data.
4.1. Trace calibration The ST-ECF aXe package for reduction of slitless spectroscopy data treats the spectral traces and wavelength solutions defined with respect to the position of the ST-ECF Instrument Science Report WFC3-2009-01 source in the direct image (Kümmel et al. 2009). The centroids of the continuum source images in the direct imaging exposures (Xref, Yref) were determined with the IRAF task imexam. These positions were assumed not to change through the duration of the remaining measurements. For the G280 grism there is a rather large offset of about 170 pixels in the y-direction between the position of the source in the direct image and the +1st order spectrum (see Figure 1). The observation of a continuum lamp spectrum with the G280 grism shows that many orders are visible and largely overlapping with each other (see Figure 1). Due to this heavy overlap, the typical approach of tracing the continuum spectrum as a function of ΔX=X-Xref in the CCD X-direction is not feasible here. Instead we used the monochromator steps observed for the dispersion calibrations (see also Section 4.2) to obtain accurate trace information. For each wavelength setting the position of the +3rd to rd orders as observed in TV3 was determined with the IRAF task imexamine (see also Figure 2). Several points are noteworthy: (a) The 0th order is weakly dispersed, spanning about 23 pixels in the y-direction for wavelengths of 200 to 915nm; (b) the +1st order (defined as the order carrying the maximum throughput in the UV and hence the one extending to decreasing x-axis pixels towards the left of the 0th order) follows a highly curved trace and overlaps with the +2nd order in x-direction for wavelengths larger than about 390nm. The same is true for the -1st and -2nd order spectra; (c) higher orders (negative and positive) strongly overlap in the xdirection and are essentially merging into a single spectrum. Hence the observation of a reasonably bright source placed near the chip centre in the x-direction affects a semi-contiguous band of pixels across the full x-extension of the detector.
In TV3 only one central position was calibrated for each chip (see Table 3). Using the trace information from the monochromator steps we determine trace solutions for the +3rd to -3rd spectral orders for wavelength between 200 and 815nm; the monochromator step at 915nm was omitted in the trace analysis since, for unknown reasons, it did not follow the general trends. Due to S/N limitations there is only reduced information for higher spectral orders available, but not considered in this ISR. The results of the trace fits are given in Tables 4 and 5, respectively.
The trace definitions are of the form (Y-Yref)=DYDX_0 + DYDX_1*ΔX + DYDX_2*ΔX2 + DYDX_3*ΔX3 + DYDX_4*ΔX4 where DYDX_0-4 are normally field dependent and given in the usual format used by the ST-ECF aXe reduction package, e.g., DYDX_1 = a0 +a1*Xref + a2*Yref + …(see also the aXe manual for more details). During TV2 several positions (5-point pattern for each chip) across the field were calibrated. However, attempts to establish a stable, field dependent solution failed due to (a) significant and complex variations of the trace shape across the field-of-view (see also Figure 11); and (b) the lack of enough calibration points to sample the field dependent variations. In conclusion, we can currently provide a calibration for the trace and wavelength solution only for one central target position on each chip.
ST-ECF Instrument Science Report WFC3-2009-01
Figure 1: Sum of a direct image (F300X) and a G280 exposure of the continuum point source. A black circle marks the position of the source on the direct image.
The prominent 0th order appears at the centre of the image (with spikes); the grism +1st and higher positive orders can be seen extending towards the left and the negative orders to the right. There is extended overlap between all orders. The full extent of the detector (4096 pixels) is shown in the x-direction while the y-axis shows a 1024 sub-array.
Figure 2: Schematic representation of traces in orders +8 to -8 as derived from monochromator steps obtained during TV2. For each order, colours visualize wavelength from pale purple to green for wavelengths of 200 to 530nm, respectively.
In order to achieve a good trace accuracy (≤1 pixel), a 4th order polynomial fit was needed for all orders apart from the 0th order where a 3rd order polynomial was sufficient. The traces show a strong curvature towards short wavelengths as can be
ST-ECF Instrument Science Report WFC3-2009-01
seen in Figure 3. Although the overall trace shapes are similar for all positive (or negative) orders, they are not identical and individual fits are made for each order.
Even for a 4th order polynomial fit, the fit residuals show a clear pattern, which could be removed by using a higher order polynomial. However, for numerical stability considerations, we decided to use the 4th order fits. Once in-orbit data is available, this decision should be reviewed.
Figure 3: 4th order polynomial fit to the trace of the +1st and -1st orders on Chip 1 as determined in TV3. Monochromator steps between 200 and 815nm are fitted.
Colours visualize wavelength from pale purple to green for wavelengths of 200 to 815nm, respectively. See also Figure 2.
ST-ECF Instrument Science Report WFC3-2009-01
The trace definitions derived in Section 4.1 were inserted into a configuration file for the spectral extraction software (aXe), and each of the monochromator spectra were then extracted using the standard aXe task AXECORE. The software automatically extracts the spectral orders as separate “beams”, defined with respect to the location of the object in the direct image. The extracted spectra produced by aXe were then analyzed using custom-built IDL scripts where the (X-Xref) location of the peak of each monochromator spot was measured by fitting a Gaussian.